Wednesday, 4 December 2013

Shale gas and methane emissions - a perspective from RealClimate.org


A major part of the debate around shale gas has focused on methane emissions and climate change. Natural gas, when burned, produces half as much CO2 as coal, so switching from coal fired power stations (still ~40% of our electricity supply) to natural gas could provide substantial emissions reductions. However, if a portion of the methane produced during extraction is allowed to leak into the atmosphere, then this could offset the CO2 gains, because methane is itself a powerful greenhouse gas.

We've seen a number of papers in recent times trying to get a handle on what proportion of methane produced from shale wells is ending up in the atmosphere, rather than in our boilers. Direct measurements at drilling sites have given low estimates, but regional overflight measurements have indicated that overall estimates might be underestimated - although with regional overflights, the source of methane (conventional gas, coal mining, shale gas, agricultural) cannot be determined.

The key question to ask is - how relevant are these estimates to global climate change, and are we seeing changes in methane emission rates impacting on the global concentration of methane in our atmosphere, and therefore on global climate change?

Who better to answer this question than the climate experts at RealClimate.org - certainly no stooges for the oil and gas industry.

So - are reported increases in methane emissions bad news for global warming? (my emphasis added)
Not really, because the one real hard fact that we know about atmospheric methane is that it’s concentration isn’t rising very quickly. Methane is a short-lived gas in the atmosphere, so to make it rise, the emission flux has to continually increase. This is in contrast to CO2, which accumulates in the atmosphere/ocean system, meaning that steady (non-rising) emissions still lead to a rising atmospheric concentration. There is enough uncertainty in the methane budget that tweaks of a few percent here and there don’t upset the apple cart. Since the methane concentration wasn’t rising all that much, its sources, uncertain as they are, have been mostly balanced by sinks, also uncertain. If anything, the paper is good news for people concerned about global warming, because it gives us something to fix.
 Also, in more general terms:
The US is apparently emitting more than we thought we were, maybe 30 Tg CH4 per year. But these fluxes are relatively small compared to the global emission rate of about 600 Tg CH4 per year. The Arctic and US anthropogenic are each about 5% of the total. Changes in the atmospheric concentration scale more-or-less with changes in the chronic emission flux, so unless these sources suddenly increase by an order of magnitude or more, they won’t dominate the atmospheric concentration of methane, or its climate impact.
I am not a climate scientist, and there are probably too many non-experts shouting their views from the rooftops, so I will pose my conclusions instead as a tentative question: are we overestimating the importance of shale gas methane emissions with respect to climate change?





3 comments:

  1. A great post JV, about time this whole CH4 leakage was contextualised.

    I've been saying this for a while now: methane is effectively 'metabolised' in the atmosphere, reacting with other materials in the presence of sunlight to form less climate-influencing substances. So whilst a more potent greenhouse gas than CO2 over a shorter timeframe, its residence and, therefore, overall impact is actually lower than CO2.

    We also shouldn't ignore the other massive sources of CH4 in the atmosphere from other sources, particularly agriculture.

    Well done for shining some light on this topic!

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  2. It doesn't add up...26 December 2013 03:49

    This study has an interesting conclusion on the impact on water resources:

    The amount of water saved by using NGCC rather than coal steam turbine plants (~100 000 af) is about 25–50 times the amount of water required (~2000–4000 af) to hydraulically fracture and produce the natural gas (700 e12 Btu) used to generate this electricity, based on 1–2 gal to extract 1 million Btu of gas in Texas and assuming a thermal energy efficiency of NGCC plants of 44% efficient based on data for Texas.

    http://iopscience.iop.org/1748-9326/8/4/045033/article#fnref-erl480476bib38

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    Replies
    1. Thanks IDAU,
      If you can hold on to your horses for a few days, I am actually drafting a new post on this and a couple of other papers related to water consumption. Cheers, JV.

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